Bringing New Life to an Old Classic Marantz Stereo Receiver With a Class D Amp Board.

Introduction: Bringing New Life to an Old Classic Marantz Stereo Receiver With a Class D Amp Board.

*this instructable takes for granted that you can solder, understand basic electronics, and have fixed electronic gear before.*

so you acquired an old stereo receiver but it has issues. there are many things that can go wrong with an old stereo receiver but one of the common issues can be blown outputs. sometimes the cost of parts is prohibitive, making it not worth the effort of trying to fix a piece back to its original design.

what to do? junk it? NO..

this instructable is intended to be an option for those that have a piece not worth fixing. vintage stereo gear can be valuable, even broken so don't just dive in till you know what you have. you may regret your actions later if by chance you had something quite valuable, even if broken. finally, this instructable is focused on transistorized stereo gear from the 70's and 80's. old tube gear is a totally different animal and not covered here. throughout this instructable an effort will be made at trying to solve some of the common ailments of old stereo gear. there's always a chance you might get lucky and what ails your unit is covered here and you are able to fix it instead of mod it.

with that out of the way, let's see what's up with this old marantz.

Step 1: A Little Diagnoses

time to hook it up and see what happens.

first thing, inspect the power cord. any signs of damage to the cord where you may be exposed to bare wire are a bad thing and need to be addressed before applying power. if cord is all good let's move on to powering it up.

what can happen? smoke, fire, sparks, or nothing at all. find somewhere safe to work in case something goes VERY wrong with your stereo. turn the volume knob down all the way, plug the power cord in, hit the power switch and watch.

smoke = bad sparks = bad fire = very bad does nothing = not so bad powered up and seems to be acting normal = doing good

if you got smoke, fire, or sparks, we are done here and that is beyond the scope of this instructable. your unit may have a catastrophic failure but may still be valuable to someone for parts. don't just trash it. put it up on craigs or something and let somebody else take a crack at it if it's beyond your abilities.

before we proceed keep in mind there are dangerous voltages inside old stereo gear. you can die if you touch the wrong thing inside an old stereo while its plugged in. if you are unfamiliar with proper safety procedures, you should seek the help of someone who is trained in working with high voltages.

Step 2: Digging Deeper

so nothing blew up. good!

did the unit power up and the lights came on? yes? then hook some speakers to it and let's see what happens. for this step it is HIGHLY recommended you use a pair of sacrificial speakers. in other words, speakers that you know work but wont be heartbroken if they get damaged. some electrical faults in a stereo unit can result in smoked speakers.

no lights? unit looks DOA? check the fuse. if its blown, replace it with a fuse of the same rating. a higher rated fuse can result in fire and more damage if there is a hard short somewhere in the unit.

a word about blown fuses. a clean break like the one shown could be from a light fault, the amp being over driven, or too much stuff was plugged into its accessory outlets if it has them. if the fuse has brown or black tinge in the glass like a burnt light bulb, there may be a dead short in the unit so expect the next fuse you put in to go out with a flash. just a warning.

Step 3: Fuse Keeps Popping on Old Discrete Transistor Amp Type

things get complicated here. if you score high on troubleshooting geek points you should have no trouble following me. if you've never fixed anything before, get a geek buddy to give you a hand.

constant blowing fuses are the result of a short somewhere. you need to track down that short. common causes are shorted power supply capacitors, shorted diodes in the power supply often caused by failed capacitors, or shorted audio output devices.

if your stereo uses large metal cased transistors that can be removed like those pictured, you may be in luck. take a sheet of paper and make a diagram of which transistors are in which location on the heatsink. the same part number that came out of a socket needs to go back into that socket. some transistor sockets are held in place by the same screws that hold the transistor down. it's like a sandwich with the aluminum heatsink between the transistor and the socket. you may have to use a finger or screwdriver to get socket in place when re-inserting transistors.

unscrew them one by one. take an ohmmeter, set it to continuity and measure from each leg to the case of the transistor, note findings then redo measurement except this time flip the polarity of the leads. a good result will be continuity to only one transistor lead in only one direction. continuity in both directions or to more than one transistor lead indicates a failed transistor and could be the cause of your blowing fuse.

when removing transistors make sure you keep all the hardware in order so you can reassemble exactly as it came out. there may be a thin mica washer that goes up against the flat part of the transistor and insulating washers for the hardware. they MUST go back in on reassembly. the white grease on the transistors is similar to whats used on CPU heatsinks and must be there. don't wipe it off.

if you have shorted transistors, there's a good likelihood your stereo may be saveable with just a new set of replacement output transistors. time to look at cost of new transistors versus modding the unit you have. modding will kill its collector value. if you do install new transistors, make sure and get a small tube of heatsink grease as the new transistors will have none on them.

if you do find shorted transistors go ahead and remove all the output transistors, make sure the back side of the sockets which may be loose and flopping around aren't shorting against anything, and power back up with a good fuse. if your unit comes to life you may be lucky and only need a set of output transistors. this is very crude diagnoses and there may be other faults but you have a good likelihood that your stereo quit working due to abuse, hence the blown transistors.

some equipment brands soldered the wires to the transistors. in this case the diagnosis method is the same but you have to unsolder the wires going to the transistors at the transistors themselves or at the board. make sure you note colors and position of colors. do NOT overheat the legs on the transistors. you can ruin an otherwise good transistor with too much heat.

if you opt to try and replace the old transistors, buy replacements only from a reputable vendor. the auction site is flooded with chinese counterfeit transistors so be careful where you spend your money.

Step 4: I Have a Big Chip in My Amp

sometime in the 80's, chip amps caught on and many manufacturers started using them. they look like whats in the picture. its basically a large chip bolted to a heatsink. often times there is an STK part number on it. this is basically an amp on a chip. it was a cost cutting measure.

theres no way to easily test these. unsolder the whole chip, make sure you didnt bridge any solder pads, power amp up and see if fuse keeps blowing. fuse stops blowing? probably a shorted STK chip. fuse keeps blowing? other issue which we will address.

stereo gear with STK's in them may prove to be harder to mod. keep that in mind. a great many STK part numbers are still available but price varies drastically. be careful of chinese counterfeits. buy only from a reputable vendor.

Step 5: Its Not the Amp Section and I Still Have a Short

still blowing fuses?

a failed power supply could be the culprit. old electrolytic caps can short out and take out the diodes in the power supply. look for bloated or leaking capacitors.

to check the power supply diodes you need to disconect one leg of the device from the circuit and check for continuity in diode test. there should be continuity in only one direction. if your amp uses a rectifier bridge, you will have to desolder the whole device to test it. it should be marked with a pinout.

blown diodes and bad caps go hand in hand.

if everything so far checks out and your still popping fuses it may be time to consider walking away from that project. you could have a bad power transformer which is basically the heart of the stereo. no power = no musical love.

Step 6: You've Got Power, Now What?

with sacrificial speakers connected, power up and let's see what you get.

unit plays fine = seller didnt bother testing things? score! unit plays only on one channel or on neither channel = you could have a blown channel (maybe bad STK or output transistors), dirty controls, missing preamp jumper, or "tape monitor" button is pushed in. unit plays but makes static noises = electronic fault or dirty controls. if using FM make sure you have an antenna connected. having no antenna will get you a noisy signal.

before condemning a unit to have a bad amp section make sure the controls are set right. if it has a tuner, set it to FM, muting off, on mono (since you will likely just be using a random wire as an antenna), and work the "tape mon" button a few times. could just be a dirty control.

look at the back, is there a set of RCA jacks labeled "pre out" and "amp in" or something to that effect? some receivers had the ability to be split from their amp sections via those jacks on the back. if your unit has those jacks, run an rca jumper from the pre out to the amp in and see if it doesn't come back to life. many a receiver has been junked because it was just missing those jumpers.

if unit has static crashes and random crackles try getting in there with contact cleaner and spraying out all the switches. give them a shot of contact cleaner and then work the switch.

the goal here is to try to bring it back to life. the last option is to mod it.

Step 7: Tried It All, I Can't Save Her Captain..

so you have dead channels you can't / don't want to fix or you have crackling noises that don't go away with switch cleaning? its time to do some further tests to see if your unit is worth modding to an aftermarket amp board.

if you are fortunate enough to have a receiver that has "pre out", this step will be made drastically easy for you. run an rca cable from the "pre out" jacks to a known good amp and see what you get. if it doesn't have preamp outputs, you need to dig into the unit.

somewhere feeding the amp section of your receiver, is an audio line. this line carries the line level audio from the preamp section (tone controls, volume control, balance control) and sends the preamp output to the audio amp section input. on some units, like the marantz at the start of this instructable, there is an actual cable that does this. on other units, it may be a trace on a circuit board.

if it's a cable, it's usually one three conductor cable. it supplies ground, left audio, right audio. sometimes pc board traces are also laid out in this format. on large receivers, you may have separate left and right amp sections which means separate left and right audio lines. you must find this line and separate it from the amp board.

take that line that's coming FROM the preamp section of the receiver and wire it up to an rca cable and plug that into a known good amp. if all is well, you should be able to hear the audio coming from the stereo you are going to mod and all its controls should work like balance, tone, volume, etc. the audio should be free of crackling, excessive hiss, or loud hum. you are essentially using the old receiver as a preamp.

if everything works as intended by using the preamp out of the receiver then you're ready to proceed. if things aren't working as they are supposed to, you may have made an error somewhere or there's something else wrong with the old receiver. go back and re-check things.

Step 8: Got Audio?

if you have nice clean audio coming out of the preamp section, you are ready to proceed with the mod. if you have random static crashes or other issues, you need to re-check your work. if your work is fine, you have other problems in that receiver and it may be time to reconsider proceeding.

let's say all is hunky dory and you have good audio coming out of the preamp line on your project receiver. great! time to plan and dig in.

1) how much space you got? do you have an amp board in mind to use? will it fit? 2) how are you going to mount the new amp board? 3) how will you power the new amp board?

space is not an issue with most old receivers. mounting is. your amp board needs to be mounted in a mechanically sound fashion. you don't want it shaking loose at some point. this calls for nuts and bolts. no double sided tape rigging here please.

Step 9: Getting Power

power for your amp can be done in different ways. you can use an SMPS (switched mode power supply), you can add a transformer and build another supply on the chassis of the receiver if space permits, or you can use what was running to old amp section as the basis for your new supply to run the new amp board.

goods and bads..

SMPS goods - standalone unit, plug and play, you can get the voltage you need right from the get go, very compact when compared to a conventional standalone supply. bads - can induce radio noise, specially if you intend to use the AM tuner section on your modded receiver.

building a separate conventional supply goods - no RF noise, less stuff to go wrong in it. bads - big and bulky, space may be an issue.

using the old power supply section to run your new amp board goods - most of it is already there bads - 99% chance the voltages are all wrong requiring some creative voltage regulation

for my build i went with an SMPS. since i was using a small 15wpc class d amp board, it didn't take much to run it. i used JB weld to epoxy the PSU in place and a tie wrap to hold it tight while the epoxy dried. the leads that power the little PSU were soldered to the back of the switched accessory outlet on the receiver. when the receiver is powered on, so is the amp board.

this receiver was originally rated at 20wpc so 15wpc is not too far from the original design. i could have easily gone the other way though and dropped a 100wpc board into the area that was originally occupied by the old amp board. the class d boards are quite small compared to their regular class a/b counterparts and require little or no heatsink in some cases.

Step 10: There's NO Common Ground Here!

this isn't a political rant, i'm referring to common ground leads feeding the speakers.

here is the one place you might have to give up a feature or two of the original receiver's design. headphones typically use three conductors, ground, left audio, right audio. the speaker selector switch on many stereo receivers is typically designed to work with this common ground setup.

all the class d amp boards i have seen so far advise against wiring for common ground on the speakers. this can actually damage the amp. because of this you may find yourself having to remove the original wiring that feeds the speaker terminals on the receiver, removing the jumpers between the ground terminals if any are present, and wiring the amp board straight to the speaker jacks on the receiver.

end result? no more working headphone jack in most cases and the speaker select switches on the front of the unit will no longer work. a few high end receivers had a separate dedicated headphone amp but not many. i consider it a small price to pay.

Step 11: Kick Back and Enjoy

if all went well, you should have a working vintage stereo receiver with a new soul. how will it sound? i'm a fan of classic american tube amps and i personally like the sound of class d amps quite a bit. they are crisp with good low end. they are pleasing to my ear until you overdrive them. in other words, they sound great till you turn them up too loud and make them distort. when they distort, they do it in a very nasty manner.

choose your class d amp board based on your listening habits. this particular marantz is going back to its owner, an old timer that just wants to spin some records. he's no party animal and 15wpc is more than enough for his needs. pick your board according to your wattage needs and you'll be fine.

into old tech? follow me on instagram as vintagetechguy to see random pics of interesting old tech.

Great article and such a gold mine for me! Have a Marantz 2215B and have realized its that - and not the gramophone (grounded to the receiver - but receiver itself is not truly grounded cause my sockets here have no ground) that is the problem.

When I spin LPs, it works for a while - but then one channel always give up - and gets quiet. Even if I swop R/L RCAs its still same speaker that gets quiet. I can kickstart it again if I pull them both out and reconnect them (with the loud grounding sound until I connected them right). After that it works for a while again. Do you know what part that might be involved in this case?

something else to check. have you cleaned the controls? there is a product called deoxit that is meant for cleaning electrical contacts. these old stereos will develop crusty connections that can cause signal to drop. get an account on tapeheads.net and post on solidstate stereo section. surely there will be pictures to show you how to do it.

the ground to power isnt a big deal. these things were made to use ungrounded power source in the USA.

does it happen with FM radio also? if yes, i would be looking for bad coupling capacitor or maybe failing transistor in audio path for that bad channel in the main preamp section.

if the problem is only happening on phono, then i would be checking the problem channel in the phono preamp only. once again my first check would be bad coupling capacitor then maybe failing transistor. nice thing is the phono preamp is fairly simple circuit. you could just replace all electrolytics capacitors in that circuit with new ones of proper value and maybe catch the problem.

Great article, cleaned up a old monster receiver Hitachi SR-2004. Cleaned the pots and switches, and plays nice but under further evaluation left side seems lacking in bass compared to right. Switched L/R source inputs, switched L/R speakers, still follows left side output of amp. Could this be a dry cap on the pre-amp section? Any thoughts or hints appreciated.

to me it sounds most definitely like a bad electrolytic in the audio path. can be found by bridging a known good cap of same or slightly higher value across good cap and seeing if audio improves. watch polarity! when you find it might as well replace its counterpart on other channel.

I recently found my receiver blown after a party. Cracked it open, and found a burnt fuse that looked like a burned out bulb with a little soot on the side. I replaced the fuse, turned on the receiver, and saw smoke coming from one of the transistors. After reassessing, I concluded that it must be the right channel controlling transistor, as when the right channels are used, the speakers produce a loud, crackly sound. Is my assumption correct? I already ordered a replacement transistor (from China, sadly), should I order one from a more reputable place? Thank you

Wrong speakers could blow the transistors if cranked up. That would result in no audio. You should check to make sure you have audio on both channels feeding the amp board. There should be a light weight shielded two conductor cable feeding the board. That cable should be the one that carries audio from the receivers preamp section into your amp.

If you fix the amp again and no audio, I'd check the feeds to the amp.

I have an old amplifier from my Dad (a realistic STA-18), and the left channel doesn't work. I've never worked on something like this before but I tried to figure out the problem. I found a blown fuse inside the amplifier that connected to the left channel on the main amp circuit board. I used a multimeter to test the 4 transistors and found one on the left channel side that didn't give a reading, so I replaced it, and put a new fuse in. Well... limited success: I can now get a faint sound out of the left channel speaker. The fuse didn't blow, but the reading off the new transistor I put in is still a bit funny (very low- changed since installed, so maybe partially burnt right away?). I measure the voltage coming out of the two speakers now: 80 mVolts from right channel (just right I'm told) and only 4 from the left channel... which is about what we're hearing.

80mV DC offset is high. should be in the 10mV or less range. this could be from leaky caps as it is an old unit. some stupid questions.. is the part that you used an exact replacement for what was in the unit? if it was, and no fuses have blown, my next step would be to hit all the controls and switches with contact cleaner. old stereo gear that has sat for a long time can always benefit from a good cleaning of its switches and pots.

Thanks for the answer!So there are two transistors on the left channel, and I replaced both of them with the same type of trasnsistor- they were shorted out and the fuse was blowing. Now, the fuse doesn't blow and they're not shorted out, and there's some volume (very faint) coming out of left channel, but not much. Also...the multimeter reading on both the new transistors goes straight down to zero within a second of touching the leads (the two right channel transistors test normally with the multimeter while in-circuit.) Does this maybe mean that a capacitor or resistor isn't working on the same amp board? Nothing is discoloured/blew out that I can tell. But what/how should I test next?

As for the contact cleaner- I will try that, BUT this amp was in fairly steady use. As far as I can tell there's no gunk or dust buildup- it all seems pretty clean. Then again.. what would I know :)

Appreciate your help! This has been driving me crazy...I've attached two pics- one of the whole interior of the receiver, one of a close-up of the main amp board, with the two transistors I replaced on the left side.

on the two output transistors you replaced, did you put the mica spacers back in? I see a cooked bias resistor too. If the bias resistor has been replied and the mica washers are back my next stop would be dried out coupling caps. Now on the transistors, they should have been two different part numbers and you did note their placement right?

Ok- I put the mica spacers back and replaces both blown bias resistors- there was some discolouration on the metal next to them that I hadn't seen. The result... no sound! Unfortunately the very weak signal is gone entirely again from the left channel. Still, thanks to you I feel I'm on the right track. Any suggestion as to what I should try next?

FYI- What seems to have happened is someone (my brother...) connected the wrong speaker setup to the left channel and blew out the two resistors, transistors and the fuse on the left channel. Those things haven all been switched out now, but clearly there's something else not right/or that was blown at the same time as the other items.

Is this the one (this is the 2nd of two, behind the two left channel transistors)- it's red, not brown, but I think it's the one you're talking about. I didn't think the black was a burn mark, just the colour marking...

that's the spacer and it's very important. the transistors are electrically isolated from the heatsink. the bias resistor is the brown one near the transistors that's burnt in the center. the one from the other channel looks ok.

Thank you for this. I have a Marantz 2420 that my Dad bought in 1970. In my first floor dorm room, it could be heard on the third floor. Beautiful piece of equipment. Not knowing what I was doing, in 1999, I hooked up my speakers (8 ohm) and my friend's truck box (4 ohm). There was a strange chirp and it gave up the ghost. It's been boxed up since then, but I recently repaired a TV and thought I might have a crack at the receiver. I'm currently on step 3. The transistors failed the continuity test. Two had two way continuity and two others had none at all. With the transistors removed and a new fuse in place, it successfully powered on. There are no good local stores for electronic components, but I ordered some that should arrive by the end of the week. This beast shall live again! Pictures to follow when it is up and running.

So.... New transistors installed, the unit comes to life. I start with all EQs centered, high and low filters and loudness off, volume all the way down, speakers connected to main, main on, remote off. Bringing the volume up slowly everything sounds fine. Tested balance and filters, all good. After a couple minutes there was some crackling noise from the right side speaker. I turned it off, and switched the speakers to the remote channel to test it, and also to identify if the crackling was a receiver or speaker problem. No sound, and right away a small puff of smoke from the right side (volume knob side). I shut it off, and reconnected to main, hoping the problem was limited to the remote channel. No such luck. Smoke, and one resistor almost immediately red hot (R727). Further inspection shows R716 and R728 are scorched. I think they were the first two puffs of smoke and R727 was about to do the same. The fuse did not blow. I think (wish/hope/pray) that the resistors started to cook off but hadn't failed so badly to burn the fuse or fry anything else.Also came across this anomaly: above R728, the board has holes and symbols for a capacitor (C713) but there's no sign of it having been there. It is present on the schematics as a 220pf 300V capacitor. Any ideas on what is cooking my resistors and what's up with this mystery capacitor?

i just noticed this post. did you ever revive it? marantz was know for overbuilding their gear. my next step in your mission would have been to check the power supply for shorted diodes before replacing burnt resistors.

id have to respectfully disagree with you. i not only tinker with old stereo gear but also collect vintage tube audio gear. audio is a very subjective hobby and ultimately one should buy what sounds good to them.

i have found some lousy class d boards but have also found a great many excellent sounding ones. in particular all the boards i have gotten from Sure Electronics in the 15wpc class and above have sounded great.

were class d falls short is when it clips. it goes from beautiful sound to cats fighting on a chalk board really fast. its very important one does not drive class d amps into clipping.

the mod that was done on this marantz is fully reversible. if you keep your old parts, there's no reason somebody cant go and undo what you did, repair or replace the original amp board, and wire it all back up the way it was.

Sure, there's no questioning on the subjective part. But Class D amps, even the better ones, have its issues - as all types os amps have, even single-ended class A tube amps.

D's are not good with very high audio frequencies (God damn you, Nyquist!) and they tend to turn things on the bass side a little muddy - just like tube amps, even the better ones, 'cause the damping factor is too low.

This is no opinion, this is something tried and tested on the road - ask a PA owner or tecnician and he/she will share his/her disgust about Class D amps with you. They still love AB or H for a reason.

Anyway, of course there are marvellous D amps out there, and if the instructable was about some cheap 80's stereo I'd totally support the "transformation". But it was a Marantz. It deserved some respect.

I owned a similar model although mine was Quadraphonic. My unit was rated at 20 watts per channel output. I burnt one of the channels out in it and it was impossible to dig into it to change the transistor due to how it was made. The thing was a total rats nest! It was OK back when I used it but I've had a lot of stereos since so it wasn't worth repairing to me. Not many consumer electronics really are. I did scrap it for salvageable parts though.

wow.. that would have been an excellent unit to drop a Sure Electronics 4x100wpc board in! i'm familiar with the quad versions of the marantz line in this instructable and they are really well suited for this mod.